The invention relates to the field of lattice-mismatched epitaxy, and in particular to the field of creating lattice-mismatched devices based on relaxed InGaAs alloys.
Most electronic and optoelectronic devices that require layers deposited by epitaxial growth utilize lattice-matched epitaxial layers, i.e. the crystal structure of the layer has the same lattice constant as that of the substrate. This lattice-matching criterion has been important in creating high quality materials and devices, since lattice-mismatch will create stress and in turn introduce dislocations and other defects into the layers. The dislocations and other defects will often degrade device performance, and more importantly, reduce the reliability of the device.
The applications of lattice-mismatched layers are numerous. In the InGaAs material system, one important composition is in the range of 20-30% In. These compositions would allow the fabrication of 1.3 .mu.m optical devices on GaAs substrates, as well as high electron mobility transistors with superior performance on GaAs substrates. Alloys in the desired composition range are lattice-mismatched to GaAs and InP substrates, and therefore usually suffer from high dislocation densities. One known method to minimize the number of dislocations reaching the surface of a relaxed, mismatched layer is to compositionally grade the material (in this case through grading the In composition) so that the lattice-mismatched is reduced over a great thickness.
Compositional grading is typically accomplished in InGaAs alloys by grading the In composition at a low temperature of growth, typically less than 500.degree. C. The dominant technique for depositing these relaxed layers in the InGaAs system has been molecular beam epitaxy (MBE). The MBE has a limited growth rate, and therefore the growth of these relaxed buffers is tedious and costly. A supply of virtual InGaAs substrates (i.e., a GaAs substrate with a relaxed InGaAs layer of high quality at the surface) would be in demand commercially, since the user could buy the substrate and deposit the device layers without having to deposit the graded InGaAs layer. To create a supply of these wafers at low cost, metalorganic chemical vapor deposition (MOCVD) offers greater potential.
There have been no successful reports of high quality relaxed graded InGaAs layers grown by MOCVD. There are fundamental materials problems with InGaAs graded layers grown in a certain temperature window. Thus, most attempts to grow relaxed layers with MOCVD have most likely failed for attempting to grow the layers under standard conditions, i.e. temperatures in the deleterious window.